1. Field of the Invention
The present invention relates to an encoder signal processor for calculating the position data of a target to be measured based on a sinusoidal analog signal with two phases having a 90-degree difference outputted in accordance with a travel of the target to be measured.
2. Description of the Related Art
It has been a common practice to adjust, inside an encoder for detecting the position of a traveling object to be measured, a signal to be processed in the encoder in order to attain high-accuracy position detection.
FIG. 20 illustrates a block diagram of a general-purpose encoder signal processor. Typically, an encoder 101 includes a sensing unit 40, an analog signal adjustment circuit 50 configured as an analog circuit, and a position detection circuit 60 configured as a digital circuit. The sensing unit 40 outputs a sinusoidal analog signal with two phases, Phase-A and Phase-B, having a 90-degree difference, in accordance with a travel of a target to be measured. The analog signal is transmitted via the analog signal adjustment circuit 50 to analog/digital (A/D) converter circuits 53a and 53b, and converted to a digital signal therein. The digital signal is then used by the position detection circuit 60 to detect position data.
The analog signal adjustment circuit 50 is designed to perform magnification power adjustment and offset adjustment while being incorporated into an encoder in the manufacturing process in order to attain high-accuracy position detection. The analog signal adjustment circuit 50 includes variable resistors 51a and 51b and analog signal processor unit 52a and 52b corresponding to the phases of a two-phase signal. A mechanical volume knob is coupled to each of the variable resistors 51a and 51b in the encoder in order to alter the resistance value of each of the variable resistors 51a and 51b. Analog signal adjustment procedure in the manufacturing process of the encoder 101 includes the steps of: 1) Inputting one or the other phase of a two-phase analog signal to the corresponding variable resistor 51a or 51b; 2) Measuring an amplitude and an offset of the two-phase analog signal outputted from the analog signal processor unit 52a and 52b by the use of an external measurement apparatus; 3) Opening the cover of the encoder; 4) Turning the mechanical volume knob to adjust the resistance value of each of the variable resistors 51a and 51b to maintain the amplitude and offset values within a specified range. The procedure aims to improve position detection accuracy of the encoder 101.
As described in the Japanese Unexamined Patent Publication No. 2003-254785, an analog signal may be converted to a digital signal and then undergo removal of an offset voltage, an amplitude difference, a phase difference and waveform distortion by way of digital signal processing to attain higher-accuracy position detection. FIG. 21 is a block diagram illustrating an encoder signal processor disclosed in the Japanese Unexamined Patent Publication No. 2003-254785. A digital circuit of the encoder 101 is provided with a position correction circuit 61, which directly applies a correction formula to the position data in digital format detected by a position detection circuit 54 to improve the position detection accuracy of the encoder 101.
As described above, there have been analog signal adjustments based on the measurement result of an analog signal, hereinafter referred to simply as the analog signal adjustment, and digital signal adjustment that is based on a digital signal produced by analog/digital conversion in order to attain high-accuracy position detection with an encoder.
However, analog signal adjustment in the manufacturing process alone cannot adequately address a drop in the amplitude or a change in the offset of an analog signal outputted from a sensing unit caused by secular changes of an encoder. Under such circumstances, it may be necessary to temporality stop ordinary use of an encoder while performing analog signal adjustment. Analog signal adjustment may be difficult depending on the encoder mounting position. The above digital signal adjustment may perform correction based on the position data detected by the position detection circuit without considering a drop in the amplitude or a change in the offset of an analog signal caused by secular changes of an encoder. This may result in insufficient correction.